Cubic feet a minuite

[cgarr]

Trying to figure out my CFM of my compressor, it takes 100 seconds to fill a 30 gallon tank which is 4 cubic feet from 70 lbs to 100 lbs of pressure, so whats the CFM rate?

Thanks

[p911dad]

The first train will get to Chicago in 3 hours and 21 minutes..

[Jim Sims]

http://www.truetex.com/aircompressors.htm

[rick-l]

I asked this on another formum also and never got any answers.

Someone tell me if I am doing this right. I have a cheap gun that needs 8.4 CFM @ 40 psi. Note: some expensive guns need 12-15. I am assuming that 8.4 is volume at 1 atm, is that correct?

Looking at the example in the link above; http://www.truetex.com/aircompressors.htm
25 gallons = 3.342245989 cu ft

At 14.7 psi 1 cu. ft. in the tank contains 1 cu. ft. of air @ 14.7 psi so the tank in the example contains 3.34

At 40 psi 1 cu. ft. in the tank contains 2.72 cu. ft. of air @ 14.7 psi so the tank in the example contains 9.09

At 85 psi 1 cu. ft. in the tank contains 5.78 cu. ft. of air @ 14.7 psi so the tank in the example contains 19.33

At 102 psi 1 cu. ft. in the tank contains 6.94 cu. ft. of air @ 14.7 psi so the tank in the example contains 23.19


So I start at 102 psi the tank contains 23.19 cu. ft. (of air @ 14.7 psi) and I use up the air at a rate of 8.42 cu ft per minute down to a pressure of 40 psi where the tank contains 9.09 cu ft.

So 23.19 - 9.09 = 14.1 cu ft and using it at 8.4 CFM it would last 1 minute and 40 seconds so this compressor is inadequate?

[rick-l]

Does that work out to 4.9 CFM?

[T77911S]

sounds like you need a girl, or maybe you have one and are incapable of wooing said girl

pirates of the Caribbean

[masraum]

Wouldn't it just be easier to look up the specs based on the make/model number of the compressor?

[red-beard]

PV=nrT

I'm going to exclude Temp and entropy just to make things easy, but it is important here, since it helps calculate the efficiency of the compressor and we always get a temp increase from the entropy.... So, assume nrT=constant

P1= ATM
P2-Starting pressure
P3=Ending pressure
Vt=Volume Tank
V2=compressed air volume P2
V3=compressed air volume P3
Atmospheric Volume in the tank at P2

V2=Vt*P2/p1

V3=vt*p3/p1

dV=V3-V2

Compressor CFM average for fill tank from P2 to P3

=(V3-V3)/t

=((Vt*P3/p1)-(vt*p2/p1))/t=Vt*(p3/p1-p2/p1)/t

=Vt*(p3/p1-p2/p1)/t

=4*(((100+15)/15)-((70+15)/15))/1.67

=4*(115/15-85/15)/1.67

=4*(7.67-5.67)/1.67=4.8 CFM

[rick-l]

Originally Posted by masraum

Wouldn't it just be easier to look up the specs based on the make/model number of the compressor?

What if you think they are all just hype designed to sell a compressor?

So if my paint gun says it needs 8.4 CFM is that 8.4 cu ft at 1 atm.?


It doesn't say what pressure just 42 psi max.

[turbo6bar]

Originally Posted by rick-l

What if you think they are all just hype designed to sell a compressor?

So if my paint gun says it needs 8.4 CFM is that 8.4 cu ft at 1 atm.?


It doesn't say what pressure just 42 psi max.

CFM at pressure. If they just list CFM, it's useless.

I wouldn't think of using a paint gun requiring 8.4 CFM @ 40psi on anything under 60 gallons and at least 10 SCFM. The potential for condensation precludes running a compressor that's marginal.

[RoninLB]

I'm border line with a 5hp 60gal tank using an estimated 10cfm Sharp mfg gun.

I shot my 1st car in sections with scuba tanks.


ps: Jim Sims site is great info.